Quenching systems for hardening workpieces of steel and other metals are of great importance in technology, because the properties of the workpieces important for their use can be considerably improved by quenching. Quenching in water or oil, as well as in salt baths or in fluidized beds, have long been known. Recently, the chilling of workpieces in a gas stream has also been used, in which case a heat treated charge in a chilling chamber is subjected to a stream of cooling gas guided in a circulating path over a heat exchanger. The applied stream of gas is provided in the form of discrete jets for affecting the workpiece surfaces that are to be chilled.
An industrial furnace be equipped with such a quenching apparatus is described in European Patent 0151 700 A2 and U.S. Pat. No. 4,653,732.
The hardness and/or toughness enhancements which are attainable by quenching the workpieces, i.e. by their rapid chilling from heat treatment temperature to room temperature, precisely depend on whether the quenching process takes place at high speed after a suitable previous temperature time course has taken place for the material of the workpiece in question. For this purpose it is necessary, during the chilling process to lead away the heat present in the workpiece at a correspondingly high heat flow density through the cooled surface. The magnitude of the attainable heat flow density in each case depends upon, among other things, the heat transfer coefficient .alpha. which is measured in terms of W/n.sup.2 K. For describing the quenching effect or intensity it is common in practice, in the hardening of steel, to utilize a characteristic value, the so called H value attributed to Grossmann (M. A. Grossmann, N. Asimov, S. F. Urban, "The Hardenability of Alloy Steel" ASM Cleveland, 1939, Pages 124 to 190). This H value, on the basis of experience, lies in a region from about 0.2 to 4 for the known quenching systems using salt baths or oil, and in a region from about H=0.9 to 4, for water quenching. Known gas quenching exists only for H values in the range from 0.1 to 0.2 (compare, for example, "Handbuch der Fertigungstechnik" Carl Hansen Verlag, Munich and Vienna, Vol. 4/2, 1987, Page 1014). Higher values up to approximately 0.2 can be obtained only with strong circulation and/or overpressure.
The utilization of the known quenching systems with liquid quenching media in the H value range of 0.2 to 4 brings with it a series of fundamental difficulties which are fully known in practice. Since y the use of a liquid as a quenching medium the intensity of quenching can be varied only slightly during the chilling in practice, which variation would frequently be desirable in order to avoid hardness cracks and changes in dimensions of the workpiece, the result is that not seldom some problems of quality arise, which lead to cost problems, because the lack of controllability of the quenching process can be compensated for only by providing costly alloy elements (in steels which are to be hardened). The remnants of quenching oil, salt or water additives adhering to the workpiece after quenching must be cleaned off the workpiece and then be discarded, thus leading to an environmental problem. Finally, the most common quenching medium, namely oil, involves a danger of fire, with the consequence that safety problems also arise which require special precautions.
The above mentioned environmental and safety problems do not arise in the known gas quenching, as they are described, for example, in the above mentioned U.S. Pat. No. 4,653,732, but nevertheless it is possible to use such known gas quenching systems, because of low quenching intensity (H&lt;0.2), only for hardening highly alloyed steels, even if the process is performed under higher than atmospheric pressure.